Mass Timber Interiors: What to Specify When Exposed CLT Is the Finish

Exposed CLT interiors use the structural panel as the finished surface of a wall, ceiling or floor. With no covering layer, the panel’s grading, finish, moisture behaviour and fire performance become part of the interior specification. This article sets out what manufacturers’ documents and research bodies state for that use. Manufacturer figures are worded as claims.

What visual quality classes do CLT manufacturers declare?

KLH produces its CLT in spruce in three surface qualities, which can be combined as required: Domestic Visible (DVQ), Industrial Visible (IVQ) and Non Visible (NVQ).^[1] According to KLH, DVQ is for visible components in residential, school or office constructions and IVQ for visible components in industrial and office buildings, and it does not recommend IVQ for residential constructions.^[1] Stora Enso’s EPD lists Non-visual (NVI), Industrial visual (IVI) and Visual (VI) quality, with surfaces always sanded on both faces.^[4] Mayr-Melnhof’s EPD names Non-visible (NVI), Industrial-visible (IVI) and Residential-visible (RVI), and Binderholz’s 2019 EPD names Wohnsicht, Industriesicht and Nichtsicht (residential, industrial and non-visible).^[5][6] We found no criteria for either of those two in the documents we read.

Swedish Wood, the Swedish forest-industry body, published a guide with Sweden’s CLT manufacturers. It says other surface qualities may be offered, but must carry different designations. Its limits apply to 95 percent of the lamellae, so up to 5 percent may slightly exceed them, and it sets an overall inspection at a viewing distance of 2 to 5 metres.^[2] KLH states that its criteria apply at delivery, to the covering layer only and to one-sided visible surfaces. For narrow sides and CNC-treated surfaces the non-visible criteria apply, and for double-sided visible surfaces KLH expects a small amount of on-site reworking.^[1]

The documents disagree on sanding, so it is a separate item to state. Stora Enso’s 2023 EPD confirms the class names and sanding but not the 2012 limits, and we do not know whether the 2012 sheet has been superseded.^[3][4] A class name alone does not fix the appearance, so request each maker’s current criteria and sample panels at the class you intend to specify.

Which finishes and protective treatments do manufacturers state?

According to KLH, both its IVQ and DVQ panels are fully sanded, and UV protection, varnished elements or other surface treatment are available on request.^[1] KLH lists factory-applied protective coatings and surface refinements. They include a colourless, matte UV-protective varnish for temporary protection against moisture, soiling and UV light on visible surfaces, and two-layer finishes on one visible side in a colourless oil, a mildly pigmented varnish or a white glaze at 50 or 100 percent.^[1] In its table KLH lists protective coatings and surface refinement on request for DVQ, but for IVQ protective coatings on request, with surface refinement not recommended.^[1] KLH also lists a water-based impregnation against soft rot, blue stain and insects that it describes as not suitable for indoor application in service class 1.^[1]

For protection before the building is closed, KLH lists a diffusion-capable self-adhesive membrane against moisture during storage, transport, assembly and the building phase.^[1] Mayr-Melnhof’s EPD states that a weatherproofing or wood preservative may be needed during transport, storage or assembly.^[5] A Canadian research report from FPInnovations says that when wetting has stained appearance members, sanding is usually the most efficient way to remove the staining before finishing or refinishing.^[14] The documents we read give no fire class or emission values for any of these coatings. For a general comparison of two timber oil finishes, see our Rubio Monocoat vs Osmo article.

How do exposed CLT surfaces behave in fire and for sound?

Reaction to fire. KLH’s declarations of performance (2025) and Mayr-Melnhof’s DOP_MMH_201 state Euroclass D-s2,d0 for CLT components, and Stora Enso’s EPD states it for timber components apart from floors.^[7][8][4] Binderholz’s 2018 declaration for BBS XL states it for timber components other than floors.^[9] Mayr-Melnhof and Stora Enso state Dfl-s1 for floors.^[8][4] We found no class stated for a coated surface in these documents.

Charring and fire resistance. Mayr-Melnhof declares charring rates that depend on the adhesive and on how many layers burn. For MUF-bonded panels where only the top layer burns, they are 0.65 mm/min for floors and roofs and 0.60 mm/min for walls, rising to 0.76 and 0.71 mm/min when more layers burn. For PUR-bonded panels the top-layer rates are 0.65 and 0.63 mm/min, rising to 1.30 mm/min for floors and roofs and 0.86 mm/min for walls. The declaration footnotes the higher floor and roof values as applying up to 25 mm of char, after which 0.65 mm/min applies up to the next glue line.^[8] KLH declares fire resistance from REI 30 to REI 240 depending on panel structure or fire-protection cladding, with design parameters in its ETA, which we did not read.^[7]

Exposed linings in a room fire. Hadden and colleagues ran compartment fire experiments with three configurations of exposed CLT. According to the abstract, the compartment extinguished itself in one test but not when the test was repeated under identical conditions, and for large areas of exposed timber lining heat transfer within the compartment prevents auto-extinction. The authors link auto-extinction to the fall-off of charred layers.^[10] The abstract does not give a permitted exposed area, and we did not read national fire rules.

A built example. At Brock Commons Tallwood House in Vancouver (Acton Ostry Architects, completed August 2017), the project factsheet states that the CLT and glulam components are fully encapsulated in three to four layers of Type X gypsum board, and that the wood structure was left exposed in the 18th-floor student lounge for demonstration purposes.^[11]

Sound. KLH’s declaration lists airborne and impact sound insulation as ETA-06/0138 annex 6 and gives no values; we did not read the ETA.^[7] A 2014 conference abstract by Logawa and Hodgson describes CLT panels as very sound-reflective and describes measuring the sound absorption of exposed CLT surfaces in several British Columbia buildings, with no numbers in the abstract.^[12] Homb, Guigou-Carter and Rabold, collecting laboratory measurements, describe impact sound as a possible weak part of CLT floors, say no reliable standardised calculation model covers the whole frequency range, and point to the dynamic stiffness of the resilient layer and the floor’s mass per unit area as influences.^[13]

How do moisture, movement and emissions affect exposed CLT interiors?

Declared limits. KLH declares delivery at 12 ± 2 % wood moisture and use in service classes 1 and 2 of EN 1995-1-1.^[7] On its web page KLH states moisture movement of 0.02 % per percentage point of moisture change in the panel plane and 0.24 % in the thickness direction.^[1] KLH adds that crack and joint formation as the panel dries to its in-service moisture cannot be prevented, that cracking noises can occur in the first years and are harmless, and that fluctuations in room climate, for example while screed dries, should be avoided, with an ideal air humidity of 40 to 60 %.^[1] It warns that white-glazed surfaces may show an irreversible contrast where cracks open.^[1]

What the guidance says to expect. Swedish Wood’s guide gives Swedish climate data: in rooms heated to 20 °C, relative humidity averages 10 to 25 % in December to February. It expects a large proportion of outer lamellae to check along their length in winter, with the checks reducing in spring, summer and autumn, and checks more likely where surface lamellae are edge-glued. It gives spruce movement across the grain of about 0.26 % per percentage point of moisture content. It adds that spruce turns more yellow over time and pine darker.^[2]

FPInnovations, a Canadian research institute, states that CLT should stay below 19 % moisture content before enclosure and reach 16 % before it is covered with a low-vapour-permeance material. It says excessive checking, cupping and warping usually result from rapid drying, over-drying or cyclic wetting and drying, in service particularly when the space is suddenly heated, and says humidity control matters in a building’s first years.^[14] A 2025 abstract from the US Forest Products Laboratory states that detailed guidance on temperature and humidity setpoints to prevent drying cracks is lacking.^[15]

Emissions. KLH and Binderholz declare formaldehyde class E1, and KLH states a formaldehyde-free adhesive.^[7][9] Mayr-Melnhof’s EPD reports two chamber tests by MPA Eberswalde: formaldehyde of 0.026 ppm to EN 717-1, and, to EN 16516 with the AgBB scheme, TVOC (C6–C16) of 290 µg/m³ after 28 days and 370 µg/m³ after 3 days.^[5] Binderholz’s EPD, issued 2019 and valid until 19 March 2024 (so now expired), reports formaldehyde of 0.02 ppm to EN 717-1 and TVOC (C6–C22) of 218 µg/m³ after 28 days, tested by TÜV Rheinland LGA Products.^[6] These are results for the manufacturers’ panels as named in their EPDs, and we did not see the test reports.

What we could not check

• We read Stora Enso’s surface limits only in a 2012 sheet hosted by a third party, and Mayr-Melnhof’s and Binderholz’s class names without criteria.
• We read no ETA annexes, so fire design parameters and sound-insulation values are not quoted.
• We could not open the University of British Columbia thesis that reports measured absorption values for exposed CLT; its repository blocked our access.
• We did not read the national fire regulations that limit exposed timber surfaces.

Frequently asked questions about exposed CLT interiors

Related reading

For what CLT is, how it is made and its declared carbon figures, see our cross-laminated timber profile. For beams and columns, see glulam. For another timber finishing question, see Rubio Monocoat vs Osmo.